diff --git a/src/fenris/pruning.py b/src/fenris/pruning.py
index 1e5cbde..0db762e 100644
--- a/src/fenris/pruning.py
+++ b/src/fenris/pruning.py
@@ -143,6 +143,24 @@ def _contains_instant(
     return bounds is not None and bounds[0] <= instant < bounds[1]
 
 
+def _monitoring_period_covers(
+    conn: sqlite3.Connection,
+    start: datetime,
+    end: datetime,
+) -> bool:
+    """Require continuous monitoring before a local-day total replaces detail."""
+    for started_at, ended_at in conn.execute(
+        "SELECT started_at, ended_at FROM monitoring_periods"
+    ):
+        period_start = _parse_sample_time(started_at)
+        period_end = _parse_sample_time(ended_at) if ended_at is not None else None
+        if period_start is None or period_start > start:
+            continue
+        if ended_at is None or (period_end is not None and end <= period_end):
+            return True
+    return False
+
+
 def _has_local_replacement(
     conn: sqlite3.Connection,
     previous: tuple,
@@ -157,7 +175,12 @@ def _has_local_replacement(
 
     start_id, end_id = previous[0], current[0]
     segment_id = current[2]
-    if start_tz and start_tz == end_tz and segment_id is not None:
+    if (
+        start_tz
+        and start_tz == end_tz
+        and segment_id is not None
+        and _monitoring_period_covers(conn, start, end)
+    ):
         known_days = conn.execute(
             "SELECT local_days.utc_start, local_days.utc_end "
             "FROM local_days JOIN local_day_segment_totals "
diff --git a/tests/test_issue_100.py b/tests/test_issue_100.py
index 18f06eb..8314c11 100644
--- a/tests/test_issue_100.py
+++ b/tests/test_issue_100.py
@@ -137,6 +137,68 @@ def test_pruning_keeps_samples_when_replacement_evidence_is_missing(tmp_path):
     conn.close()
 
 
+def test_later_same_day_activity_does_not_replace_gap_interval(tmp_path):
+    store_path = tmp_path / "unrelated-local-activity.db"
+    conn = init_store(store_path)
+    gap_start = datetime(2026, 9, 1, 0, 0, tzinfo=timezone.utc)
+    gap_end = gap_start + timedelta(minutes=10)
+    later_end = gap_start + timedelta(minutes=20)
+    now = datetime(2026, 9, 30, tzinfo=timezone.utc)
+    conn.execute(
+        "INSERT INTO controller_segments (id, opened_at, identity_key) "
+        "VALUES (1, ?, 'test')",
+        (gap_start.isoformat(),),
+    )
+    for sample_time, written, read in (
+        (gap_start, 1000, 100),
+        (gap_end, 2000, 200),
+        (later_end, 2100, 210),
+    ):
+        conn.execute(
+            "INSERT INTO samples (ts, device, bytes_written, bytes_read, segment_id, local_tz) "
+            "VALUES (?, '/dev/nvme0', ?, ?, 1, 'UTC')",
+            (sample_time.isoformat(), written, read),
+        )
+
+    # UTC aggregates can replace the old counter pair, but the local-day
+    # summary contains only the later B->C interval, after monitoring resumed.
+    conn.execute(
+        "INSERT INTO hour_observations (hour, bytes_written_delta, bytes_read_delta) "
+        "VALUES ('2026-09-01T00:00:00+00:00', 1000, 100)"
+    )
+    conn.execute(
+        "INSERT INTO day_aggregates (day) VALUES ('2026-09-01')"
+    )
+    conn.execute(
+        "INSERT INTO monitoring_periods (started_at, ended_at) VALUES (?, ?)",
+        (gap_end.isoformat(), later_end.isoformat()),
+    )
+    last_sample_id = conn.execute("SELECT MAX(id) FROM samples").fetchone()[0]
+    conn.execute(
+        "INSERT INTO local_days "
+        "(local_date, tz_name, tz_offset, utc_start, utc_end, bytes_written, "
+        " bytes_read, activity_intervals, activity_precision, last_sample_id) "
+        "VALUES ('2026-09-01', 'UTC', '+00:00', ?, ?, 100, 10, 1, 'measured', ?)",
+        (
+            gap_start.isoformat(),
+            (gap_start + timedelta(days=1)).isoformat(),
+            last_sample_id,
+        ),
+    )
+    local_day_id = conn.execute("SELECT MAX(id) FROM local_days").fetchone()[0]
+    conn.execute(
+        "INSERT INTO local_day_segment_totals "
+        "(local_day_id, segment_id, bytes_written, bytes_read, activity_intervals) "
+        "VALUES (?, 1, 100, 10, 1)",
+        (local_day_id,),
+    )
+    conn.commit()
+
+    assert prune_old_samples(conn, now) == 0
+    assert conn.execute("SELECT COUNT(*) FROM samples").fetchone()[0] == 3
+    conn.close()
+
+
 def test_pruning_keeps_oldest_unpaired_sample_as_successor_anchor(tmp_path):
     store_path = tmp_path / "anchor.db"
     conn = init_store(store_path)